One of the usages of communication networks, such as cable television, the Internet and the like, is the distribution of movies in the form of video streams. Video streams are formed from a sequence of frames (i.e., still images), with corresponding display times. The transmission of video streams requires wide bandwidth and therefore video streams are generally compressed before transmission. Various compression methods, also referred to sometimes as encoding methods, have been defined. One popular compression technique is the MPEG-2 compression method. Another technique, generally considered to give better compression ratios is the H.264 method.
In some cases it is desired to change the compression of a video stream along a path from a server to a client, for example in order to match it with the capabilities of the client and/or with the network conditions. Such changing of the compression is often referred to as transcoding.
US patent publication 2007/0121651 to Casey et al., published May 31, 2007, the disclosure of which is incorporated herein by reference, describes a network based converter, which is adapted to convert multimedia signals between various formats, for example between MPEG-2 and H.264 formats, on the fly.
The compression of video streams typically involves a first stage of identifying information, of a sufficient quantity, which can be dropped with minimal loss to the image quality and a second stage in which the compression is performed based on the information from the first stage. Depending on the compression method, in the first stage, parameter values indicating how each frame is to be represented in the compressed form are selected. For example, quantization parameter values indicate how many bits are to be used to represent the data of each frame or of portions thereof.
The first stage of the compression is generally processing power intensive, and requires expensive hardware for real time, on-the-fly compression. Cheaper hardware can sometimes be used by selecting compression parameter values from a smaller set of possibilities, but this may result in less efficient compression.
FIG. 1 is a schematic illustration of a video delivery system 100 of the prior art. An encoder 102 prepares a compressed video stream in accordance with a first compression format. The compressed video stream is transmitted over a network 108 to a transcoder 110, which transcodes the video stream into a second size and/or format, for transmission to a client 112. Transcoder 110 generally comprises a decoder 120 which partially or entirely decodes the compressed video stream; a parameter selector 122, which selects parameter values for the transcoding; and an encoding engine 124, which performs the actual compression.
U.S. Pat. No. 6,542,546 to Vetro et al., titled: “Adaptable compressed bitstream transcoder”, the disclosure of which is incorporated herein by reference, describes a video transcoder adapted to operate in a plurality of transcoding modes. A manager of the transcoder dynamically selects one of the transcoding modes to be used, according to the content of the video stream transcoded and the network conditions.
US patent publication 2005/0232497 to Yogeshwar et al., published 20 Oct. 2005, the disclosure of which is incorporated herein by reference, describes a transcoding technique for translating from a first compression format to a second compression format. The transcoding technique uses compression parameters of the first format in selecting compression parameters of the second format. In one embodiment, the transcoding is performed from MPEG-2 to H.264.
US patent publication 2001/047517 to Christopoulos et al., published Nov. 29, 2001, the disclosure of which is incorporated herein by reference, describes a transcoding method in which a video stream is provided to a transcoder with transcoding hints. The transcoder encodes the video stream to fit onto a channel leading to the client, based on client capabilities and link characteristics of the channel, using the provided hints.
The methods referred to hereinabove may reduce the complexity of the transcoder to some extent, but due to the increasing complexity of advanced compression methods, transcoders are still quite complex and expensive.
Other patent publications which may be of interest include US patent publication 2005/0276580 to Zacek, published 15 Dec. 2005, EP patent application 1,069,770 to Tomson Licensing S.A., published 17 Jan. 2001, EP patent application 1 439 707 to Broadcom Corporation, filed 19 Jan. 2004, and US patent publication 2007/0147440 to Song et al., published Jun. 28, 2007, the disclosures of all of these are incorporated herein by reference.